Lessons · JavaScript · !! makes a boolean
Forcing a boolean
!!x converts any value to true or false by negating twice. It is the same conversion as Boolean(x).
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
A function documented to return a boolean should not return a string or an object that happens to be truthy. !! makes the type honest.
How to think about it
Use !! or Boolean() to normalise, never to validate: it reports truthiness, and '0' is truthy. Validate with the real test first.
Worked example
console.log(!!"false");true: a non-empty string is truthy.
console.log(!!0, !!"");false false.
console.log(!!{}, Boolean({}));true true: !! and Boolean() are the same conversion.Your turn
A boolean for whether the list has items.
const hasItems = items.length;
Solve one with the tests running
The trap
!!'false' is true. Double-bang does not parse text; it asks whether the value is truthy.
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